1. Field of the Invention
The present invention relates to a screw compressor provided with a built-in volume ratio adjusting valve and a volume control slide valve.
2. Description of the Related Art
FIG. 3 shows a conventional screw compressor provided with a built-in volume ratio adjusting valve 11 and a volume control slide valve 12. This screw compressor comprises: a casing 15 provided with a suction port 13 and a discharge port 14; a pair of intermeshing, counterrotating screw rotors 16 disposed within and journaled on the casing 15; a cylinder 22 fixed to the casing 15, provided with a partition wall 17 held in place with snap rings and partitioning the interior of the cylinder 22 into a first chamber 18 on the near side with respect to the rotors 16, provided with an inlet port 20X and an outlet port 20Y for the working fluid, and a second chamber 19 on the far side with respect to the rotors 16, provided with an inlet port 21X and an outlet port 21Y for the working fluid; a first piston 24 fitted in the first chamber 18 for axial sliding motion; a second piston 25 fitted in the second chamber 19 for axial sliding motion; a first piston rod 26 having one end joined to the first piston 24 and the other end operatively connected to the built-in volume ratio adjusting valve 11 to move the inner volume adjusting valve 11 in a space between the rotors 16 and the inner surface of the casing 15; a second piston rod 27 having one end joined to the second piston 25 and the other end operatively connected to the volume control slide valve 12 to move the volume control slide valve 12 in a space between the rotors 16 and the inner surface of the casing; a stopper 28 formed integrally with the casing 15 to limit the backward movement of the built-in volume ratio adjusting valve 11. The second piston rod 27 is extended through the first piston rod 26 for sliding movement relative to the first piston rod 26. The volume control slide valve 12 is able to move in the operating range of the built-in volume ratio adjusting valve 11 or in the extension of the operating range of the built-in volume ratio adjusting valve 11. The backward movement of the volume control slide valve 12 is limited by the built-in volume ratio adjusting valve 11. In a state shown in FIG. 3, the first piston 24 is held immovable by spacers 29. The spacers 29 are removed when built-in volume ratio adjustment is necessary.
The adiabatic efficiency of the screw compressor reaches a maximum when EQU Vi.sup..kappa. =Pd/Ps
where Vi=V.sub.1 /V.sub.0, V.sub.1 is theoretical maximum thread volume, V.sub.0 is theoretical minimum thread volume at the opening of the outlet port, Pd/Ps is external pressure ratio, Pd is discharge pressure, Ps is suction pressure and .kappa. is the ratio of specific heat.
When the built-in volume ratio of the compressor needs to be increased to achieve the maximum adiabatic efficiency, the built-in volume ratio adjusting valve 11 is moved to the right, as viewed in FIG. 3, and, when the built-in volume ratio needs to be decreased for the same purpose, the built-in volume ratio adjusting valve 11 is moved to the left. For full load operation, the volume control slide valve 12 is placed contiguously with the built-in volume ratio adjusting valve 11 as shown in FIG. 3 to compress all the sucked gas and to discharge the same through the discharge port 14. For partial load operation or no-load operation only the volume control slide valve 12 is moved to the right so that a gap is formed between the built-in volume ratio adjusting valve 11 and the volume control slide valve 12 to return part of or all the gas sucked through the suction port 13 to the suction port 13 without compressing the gas.
When the screw compressor is operated under a condition where the suction pressure and the discharge pressure do not vary very much, the built-in volume ratio need not be adjusted by the built-in volume ratio adjusting valve 11. Accordingly, the spacers 29 are placed on both sides of the first piston 24 in the first chamber 18 to hold the first piston 24 immovable and to hold the built-in volume ratio adjusting valve 11 at a fixed position. This conventional screw compressor, however, has a problem that a large force acting on the built-in volume ratio adjusting valve 11 from the side of the rotors 16 is transmitted through the first piston rod 26, the first piston 24, the spacer 29 and the partition wall 17 and acts on the snap ring 23, and hence the snap ring 23 is liable to be damaged.